Personally I think the Mandrake models are much nicer than the Wrack ones, and as they are so similar in game terms (CCW, minimal armour, MEQ physique) that I would use them as proxies with little modification. As long as your opponent doesn't object, and you aren't using both unit types at the same time, no problem.
Mind you, I don't play in GW stores or Tourneys, mainly because I love proxy armies and models. Don't be a slave to GW! Freestyle!